The present invention concerns a device for moving and positioning an element of a machine.
Conventional devices for moving and positioning an element of a machine comprise a hydraulic jack comprising two units, a first unit constituted by a hollow body and a second unit constituted by a piston separating the inside part of the body into two compartments into each of which one of the output ducts of a servo-valve leads, one of the two units of the jack being mobile and fast with the element of the machine to be moved.
It has been contemplated to provide a servo-valve which comprises a first and second pressurized fluid output duct, a fluid supply duct and a first and second fluid return duct, a mobile valve shaft controlled by a system of electromagnets and return springs and capable of assuming two operating positions and a rest position. In the rest position of the valve shaft, no fluid is sent into the output ducts. When the electromagnets are under tension, the valve shaft may assume either of the operating positions corresponding to the putting into communication of either of the output ducts with the supply duct. The de-energizing of the electromagnets is effected automatically as soon as the element of the machine has reached the end of its stroke.
The above-described device has several drawbacks. Firstly, it is difficult to adjust the speed of movement of the mobile unit of the jack.
Moreover, when the mobile element starts off, (when the electromagnet is put under tension) and at the end of the stroke, (when the tension of the electromagnet is shut off), the movement of the jack is abrupt, this being detrimental to the proper operation of the machine.
When the tension of the electromagnet is shut off, the jack is not blocked and a drift of the element of the machine occurs.
The device contemplated by the present invention for moving and positioning an element of a machine and not having these above-mentioned drawbacks comprises, a servo-valve provided with a first and second pressurized fluid outlet duct, a fluid supply duct and a first and second fluid return duct and a hydraulic jack comprising two units one of which is mobile in relation to the other, a first element constituted by a hollow body and a second element constituted by a piston separating the inside part of the hollow body into two compartments into each of which leads one of the output ducts of the servo-valve, the mobile unit of the jack being fast with the element of the machine to be moved and positioned and the device is characterized in that the hydraulic servo-valve comprises,
firstly, a cylindrical sleeve inside which slides between two end positions a cylindrical valve shaft provided with shoulders separating the inside part of the sleeve into several chambers, including a distribution chamber comprised betweed a first and second shoulder into which the fluid supply duct leads, a first fluid return chamber comprised between the said first and a third shoulder into which the first fluid return duct leads, a second fluid return chamber comprised between the said second and a fourth shoulder into which the second fluid return duct leads;
secondly, a first and second cylindrical bore drilled in the wall of the sleeve and whose openings on the inside surface of the sleeve may be stopped up by the said first and second shoulders whose thickness is nearly the same as the diameter of the said openings, the said first and second pressurized fluid output ducts leading into the said first and second bores, the pressure of the fluid in these output ducts being a function of the degree of stopping up of the openings in the said bores,
thirdly, two control compartments whose volumes vary in opposite proportions when the valve shaft moves,
fourthly, two control ducts connected with a hydraulic control circuit, each of the control ducts leading into one of the control compartments, and the device is characterized moreover in that the valve shaft of the servovalve is coupled by coupling means to the mobile unit of the jack.
It is thus possible to vary with great accuracy the speed of movement of the valve shaft in both directions by dosing the pressure of the fluid in either of the control ducts.
By means of the servo-valve, the jack is braked at the end of its stroke and any difference in relation to its theoretical stopping position generates an effort which tends to bring the jack back to its original position.
Moreover, by varying the flow in the control ducts, the action of the servo-valve may be controlled and more particularly, a start without a jerk of the jack may be obtained.